A eukaryotic cell goes through a series of events in the cell cycle resulting in replication and proliferation. The cell cycle consists of four distinct phases: G1 phase which is a quiescent phase from the end of the previous M phase till the beginning of DNA synthesis; S phase when DNA replication occurs; G2 phase when significant protein synthesis occurs in preparation of cell mitosis (the G1, S and G2 phases being collectively known as an interphase); and M phase when nuclear division (i.e., chromosomes separate) and cytoplasmic division (i.e., cytokinesis) occur. As these events are repeated, cell replication and proliferation are accomplished.
Cell cycle check points are control mechanisms that ensure the fidelity of cell division in eukaryotic cells. These check points verify whether the process at each phase of the cell cycle has been properly completed before progression into the next phase. For example, if the cells are damaged or exposed to radiation, the cell cycle may be interrupted at three check points during the oncogenesis: the G1 check point for blocking the progress from the G1 phase to the S phase; the S check point for delaying the progress of the S phase; and the G2 check point for blocking the progress from the G2 phase to the M phase (Kastan, M. B. Nature 410: 766-7, 2001).
Such delicate regulation of the cell cycle is controlled by various regulatory molecules, the most important of which is cyclin-dependent kinase (CDK). CDKs couple with regulatory proteins called cyclins that are specifically expressed at each phase of the cell cycle to form functional units, resulting in the generation of various combinations of cyclin-CDK complexes specifically activated at each phase of the cell cycle. Upon receiving a pro-mitotic extracellular signal, the cell proceeds to the S phase. Specifically, the cyclin D-CDK2 or cyclin D-CDK6 complex is activated first, the cyclin E-CDK2 complex is next activated upon entering S phase, and then, cyclin A interacts with CDK2 to carry out the cell cycle progression during late G1 and early S phases.
As indicated above, cell cycle progression is regulated by the various cyclins and kinases interacting therewith, and the coupling with CDK inhibitory factors, such as CDK4 inhibitor (INK4) and CDK interacting protein/kinase inhibitory protein (CIP/KIP) family, plays an important role in cell cycle regulation (Balomenos, D. and Martinez, A. C. Immunol. Today 21: 551-5, 2000). Further, ataxia telangiectasia mutated (ATM), which is a serine/threonine protein kinase of the phosphatidylinositol 3-kinase related kinases (PIKK) family, has been found to control cell-cycle check points in response to DNA damage or oncogenic signals, thereby ensuring genomic integrity and stability. ATM is necessary for the phosphorylation and activation of downstream factors, such as p53, murine double minute 2 (MDM2) and BRCA1 (Lu, S. et al., Carcinogenesis 27: 848-55, 2006). For instance, if the cells receive an oncogenic signal, such as damage to the double-strand DNA, ATM activates target proteins that induce cell cycle arrest and apoptosis, resulting in the regulation of gene transcription and DNA repair (Abraham, R. T. Nat. Med. 11: 257-8, 2005).
It has been found that p18 relating to ATM acts as a tumor suppressor in mice and humans. p18 deficiency or failure increases susceptibility to cancer by suppressing apoptosis of cells with DNA damage or mutations, thereby leading to malignant transformation of cells (Abraham, R. T. Nat. Med. 11: 257-8, 2005). In previous studies using p18 knock out mice, p18 homozygous knock out mice caused embryonic lethality, while p18 heterozygous knock out mice showed high susceptibility to various tumors including liver cancer, breast cancer, lung cancer, and the like (Park, B. J. et al., Cell 120: 209-21, 2005). p18 is transported into the nucleus and activated in response to DNA damage, where an increase in p18 expression leads to the phosphorylation and activation of p53 (French, J. E. et al., Carcinogenesis 22: 99-106, 2001; Ide, F. et al., Am. J. Pathol. 163: 1729-33, 2003), which is another tumor suppressor that controls cell proliferation and death. In contrast, p18 depletion inhibits the expression of p53 (Park, B. J. et al., Cell 120: 209-21, 2005).
The tumor suppressor gene p18 is located on chromosome region 6p24-25, where a loss-of heterozygosity (LOH) region was found in lymphoma (Baumgartner, A. K. et al., Lab. Invest. 83: 1509-16, 2003). It has been suggested that LOH in this chromosome is responsible for the lower expression of p18. According to recent studies, reduced levels of endogenous p18 have generally and frequently been detected in various human cancer cell lines as well as primary tissues, suggesting that p18 is a rate-limiting factor in the mechanism for ATM-mediated p53 activation, as well as a haploinsufficient tumor suppressor (Park, B. J. et al., Cell 120: 209-21, 2005).
Based on the fact that p18 is a potent tumor suppressor which directly interacts with ATM to activate p53 in response to oncogenic signals such as DNA damage (Savitsky, K. et al., Hum. Mol. Genet. 4: 2025-32, 1995) and is an attractive target protein as a haploinsufficient tumor suppressor involved in the signaling pathway of cell-cycle checkpoints including ATM and p53 (Kastan, M. B. Nature 410: 766-7, 2001; Balomenos, D. and Martinez, A. C. Immunol. Today 21: 551-5, 2000; Abraham, R. T. Nat. Med. 11: 257-8, 2005; Park, B. J. et al., Cell 120: 209-21, 2005), the present inventors have endeavored to develop new anticancer agents.
Meanwhile, small molecules derived from synthetic compounds or natural compounds can be transported into the cells, whereas macromolecules, such as proteins, peptides, and nucleic acids, cannot. It is widely understood that macromolecules larger than 500 kDa are incapable of penetrating the plasma membrane, i.e., the lipid bilayer structure, of live cells. To overcome this problem, a macromolecule intracellular transduction technology (MITT) was developed (Jo et al., Nat. Biotech. 19: 929-33, 2001), which allows the delivery of therapeutically effective macromolecules into cells, making the development of new drugs using peptides, proteins and genetic materials possible. According to this method, if a target macromolecule is fused to a hydrophobic macromolecule transduction domain (MTD) and other cellular delivery regulators, synthesized, expressed, and purified in the form of a recombinant protein, it can penetrate the plasma membrane lipid bilayer of the cells, be accurately delivered to a target site, and then, effectively exhibit its therapeutic effect. Such MTDs facilitate the transport of many impermeable materials which are fused to peptides, proteins, DNA, RNA, synthetic compounds, and the like into the cells.
Accordingly, the inventors of the present invention have developed a method of mediating the transport of tumor suppressor p18 into the cells, where cell permeable p18 recombinant proteins are engineered by fusing a MTD to the tumor suppressor p18. Such cell permeable p18 recombinant proteins have been found to efficiently mediate the transport of tumor suppressor p18 into the cells in vivo as well as in vitro and can be used as anticancer agents for treating p18 deficiency or failure occurring in various human cancers.